The present invention relates to multi-transmit antenna wireless communication and may be more beneficial, in some embodiments, with pulsed data communications with coded signals superimposed on discrete separated-in-time radio frequency pulses such as some military frequency hopping data links or aircraft transponder systems.
When two radio antennas are used to transmit a signal (i.e. there are two sources of the radio waves), these waves interact with each other. The principle of superposition says that the resulting wave is the sum of the two individual waves. This phenomenon is generally described as interference.
When the crests of the two waves overlap, the superposition wave reaches a maximum height. This height is the sum of their amplitudes (or twice their amplitude, in the case where the initial waves have equal amplitude). The same happens when the troughs overlap, creating a resultant trough that is the sum of the negative amplitudes. This sort of interference is called constructive interference, because it increases the overall amplitude.
Alternately, when the crest of a wave overlaps with the trough of another wave, the waves cancel each other out to some degree. If the waves are symmetrical (i.e. the same wave function, but shifted by a phase or half-wavelength), they will cancel each other completely. This sort of interference is called destructive interference.
In the past, some prior art radio communication systems and methods required mitigating multiple-source (multi-antenna) problems arising from overlapping zones in some types of single-frequency-channel terrestrial mobile communications systems. In those systems, coverage is extended to a larger geographical area by providing multiple transmitting sites at geographically diverse locations that provide overlapping zones of coverage. Since these kinds of systems use the same nominal transmit frequency, the possibility of wave interference problems exist in the regions of zone overlap, which could (in the case of destructive interference) prevent the receiving station from receiving the signals from any site. It is well known in the art that introducing slightly different characteristics to each transmitted signal will cause the receiving equipment typically used in these systems to process the signals from the “best” source. These kinds of systems typically rely on the “FM Capture” or another non-linear demodulation effect wherein a receiver that accomplishes frequency demodulation tends to output the demodulated signal of the strongest signal of a multiple signal input at a disproportionally higher signal-to-noise ratio (SNR) than what the input SNR would suggest. The nature of the differing characteristics of the other sources is chosen to result in manageable level of interference to the receiver. Some examples include: In U.S. Pat. No. 4,032,846, the differing characteristic is that a slight but differing frequency offset (from the same nominal transmit frequency) is used at each transmitting site. In U.S. Pat. No. 4,490,830, the differing characteristic is that data signals input into the modulators are adjusted to have differing phase shifts at each transmitting site. In U.S. Pat. No. 4,223,405, the differing characteristic is that data signals are additively combined with specially chosen deterministic signal patterns prior to final modulation step with the deterministic signal patterns chosen to be different at each transmitting site. In U.S. Pat. No. 4,570,265, the differing characteristic is that data signals are additively combined with random signal patterns prior to final modulation step with the random signals different at each transmitting site.
While these prior art techniques did provide utility in the past, they do not work as well where the receiving equipment cannot be expected to rely on the “FM Capture” or another non-linear demodulation effect. Moreover, generally in a class of higher data rate signals using complex modulations, a non-linear demodulator is typically not possible and the receiver must process the full signal as received.
Consequently, there exists a need for improved systems and methods for reducing a static nature of interference nulls in multi-transmitting antenna pulsed data communication systems.